Homogeneous polarized light is used in a variety of different applications. For example, homogeneous polarized light is used in microscopy to improve the visibility of objects that are not easily seen with conventional microscopes. Conventional microscopes with crossed polarizers, phase contrast microscopes and Differential Interference Contrast microscopes all use homogeneous polarized light. These microscopes produce images which transform round-trip optical path differences or local anisotropy in the sample to intensity variations in the image. Microscopes rely heavily on the design of the illumination, or condenser, system. For the most demanding applications, including nanoscale imaging, the illumination may employ a laser beam adapted for telecentric scanning across the object. When the scattered laser light is collected and detected, it may be converted into an electronic image. If the detector is situated behind a pinhole conjugate to the object plane, the detection is said to be confocal. By suitable light scanning and sample translation, confocal detection may provide high resolution, three dimensional imaging of biological samples. It may also provide precise characterization of reticles or precise measurement of printed line widths in semiconductor lithography. In many cases, it is desirable to use a polarized laser as the illumination source. The components and operation of these microscopes are well known as set forth in, “The Principles of Scanning Confocal Microscopy” by T. H. Wilson, which is incorporated herein by reference.
However, inhomogeneous polarized light has not been considered for use in many applications, including lithography and optical imaging systems, such as microscopes for the inspection of semiconductor wafers, phase shift masks and reticles. Although optical beams or light having radial or azimuthal inhomogeneous polarization have been produced, there are problems with the resulting optical beams or light.
An optical lithography system is one which uses light to transfer a prescribed pattern to a photoresist film in contact with a semiconductor wafer or similar substrate. For image-based lithography, an entire pattern is transferred in a single exposure. For scanning or direct write lithography, the pattern is sequentially applied, image point by image point, or image line by image line. For scanning lithography, a polarized laser is often used.
These, and similar, optical systems frequently employ polarized illumination; the design specifications generally require a pupil polarization that is either homogeneous (that is, a beam with a polarization state which is uniform throughout the pupil) or assembled from a collection of homogeneous segments. One such microscope is the Pol-scope, originally developed at the Woods Hole Institute and recently commercialized.
In recent years it has become clear that pupil illumination with a polarization which varies spatially and in a continuous manner throughout the pupil offers many advantages, including higher resolution and higher longitudinally polarized fields at the focus of the condenser. Youngworth et al., in co-pending U.S. patent application Ser. No. 09/759,913, have disclosed the use of such fields in microscopy and inspection, and have described an interferometric method of converting an ordinary (e.g. linearly) polarized beam into an inhomogeneously polarized beam such as a cylindrical vector beam. The entire disclosure therein is hereby incorporated by reference. This method suffers from the tendency of Mach-Zehnder and Twyman Green/Michelson interferometers to drift, requiring regular adjustment to maintain the quality of the beam in the pupil. It also requires laser beams of high coherence, making the use of pulsed lasers and semiconductor lasers difficult.
Other methods of converting ordinary homogenously polarized beams into inhomogeneously polarized beams exist. For example, lasers, such as the concentric-circle-grating surface-emitting (“CCGSE”) semiconductor laser, can be used to generate azimuthally polarized light. Unfortunately, it is not easy to control which of the many possible azimuthal modes light emitted by the CCGSE laser will have. As a result, the azimuthally polarized light produced using CCGSE lasers are of little use.
Spiral wave plates, diffractive elements in interferometers, and liquid crystal spatial light modulators and fibers, have also been used to produce optical beams with inhomogeneous polarization. Each of these are either expensive and difficult to fabricate, or produce beams of inferior quality.